Subsurface well safety valves are known. Tubing mounted flapper type safety valves are traditionally assembled by fitting a flapper, hinge pin, return spring and hinge pin retainer into a seat. The assembly is then installed into a pressure housing. This type of flapper safety valve has disadvantageous effect on the maximum internal bore and minimum outer diameter of the valve which can be achieved. This is a very important consideration for the oil industry since, in general, it is desired to maximise production, (ie maximise internal bore size) but achieve this inside as small a casing line as possible (thereby saving cost on casing).
Many valve manufacturers have sought to overcome the aforementioned disadvantage by designing curved flapper valves, which are again installed within a pressure housing as hereinbefore described. However, despite extensive efforts by many manufacturers, curved flapper valves have been found to suffer from a number of problems. For example, increased manufacturing cost vis-a-vis traditional flapper valves, and as yet unexplained failure phenomenon.
It is an object of the present invention to obviate or mitigate the aforementioned problems/disadvantages in the prior art.